The present invention relates to rotating anode x-ray tube technology and is particularly related to apparatus that improves cooling and reduces heating of x-ray tube bearings. The invention also improves the ability of the x-ray tube bearing assembly to handle mechanical loads associated with larger rotating anodes and Computed Tomography (CT) systems.
Typically, an x-ray tube housing assembly includes an x-ray tube having an envelope made of metal or glass which is supported within an x-ray tube housing. The x-ray tube housing provides electrical connections to the x-ray tube supported within. The housing is filled with a fluid that surrounds the envelope, such as oil, to aid in cooling the x-ray tube by absorbing heat radiated from internal components of the x-ray tube.
In FIG. 1, a prior art x-ray tube 120 is schematically shown illustrating a common bearing assembly construction that limits bearing cooling effectiveness and bearing size, thereby limiting the thermal and mechanical loading of the bearings. The x-ray tube 120 includes a cathode assembly 122, an anode assembly 124, and an envelope 126. A housing 128 encloses the x-ray tube 120 and is filled with a cooling oil, or other suitable medium, which surrounds the tube 120.
The cathode assembly 122 includes a cathode focusing cup and at least one cathode filament. A support bracket mounts the cathode cup within the envelope. Electrical conductors are attached to the focusing cup and cathode filament. The conductors provide an appropriate source of electrical energy to each of the cup and filament respectively.
The anode assembly 124 includes a circular anode disk 130 that is mounted on a stem 132 in a conventional manner. A typical annular target area is located about the peripheral edge of the anode disk. The stem 132 is attached to a bearing shaft 133 which defines inner bearing races 134,136. An outer bearing member 146 is frictionally received in a high purity copper bearing housing 148. Outer bearing races 142, 144 are formed in the outer bearing member 146. A plurality of ball or other bearing members 140 are received between the inner bearing races 134, 136 and the outer bearing races 142, 144. The bearing housing 148 is attached to a non-electrically conducting portion of the envelope 128 with a bolt 125.
An induction motor 150 rotates the anode assembly 124. The induction motor includes a stator having driving coils 152 which are positioned outside the vacuum envelope 126. A rotor assembly 154 inside the envelope encloses the bearing assembly and is operatively attached to the anode stem 132. The rotor assembly 154 includes a cylindrical sleeve 156 attached in a known manner to a generally cylindrical support member 155 connected to the stem 132. Typically the sleeve 156, is formed of a thermally and electrically conductive material such as copper. When the motor is energized, the rotor assembly 154 rotates within the envelope 126.
In order to produce x-rays, the cathode filament is heated with an electric current such that thermonic emission occurs thereby producing a cloud of electrons. A high electrical potential, on the order of 100-200 kV, is applied across the cathode assembly and anode assembly. This potential causes the emitted electrons to flow from the cathode through the evacuated region in the envelope to the target on the rotating anode. The cathode cup focuses the electrons into a beam that is directed onto the annular target track. The electron bean impinges the target with sufficient energy that x-rays are generated.
The electron beam produces substantial heat when striking the anode during x-ray generation. Rotating anode configurations have been adopted to distribute the thermal loading created during the production of x-rays. Each portion along the path of the annular target portion becomes heated to a very high temperature during the generation of x-rays and is cooled as it is rotated before returning to be struck again by the electron beam. In many high powered x-ray tube applications, such as Computed Tomography (CT), the generation of x-rays often causes the anode assembly to be heated to a temperature range of 1200-1400.degree. C., for example.
During operation of the x-ray tube, the x-ray tube is cooled by use of oil or other cooling fluid that surrounds the evacuated envelope and flows within the housing. The oil serves to absorb heat radiated by the anode assembly through the envelope. However, a portion of the heat radiating from the anode 130 is also absorbed by the rotor and bearing assembly. In addition, some heat is conducted from the anode 130 along the stem 132 into the bearing assembly. Some of the heat in the bearing assembly is radiated through the envelope 126 and a portion of the heat is conducted to the end of the bearing housing 148 near the mounting bolt 125. These mechanisms for removing heat from the bearing assembly are inefficient and result in bearing assembly component temperatures higher than desired.
Present x-ray tubes, as shown in FIG. 1, have a number of components surrounding the bearings such as the bearing housing and the rotor for the induction motor. These components (i) limit the efficiency of heat removal from the bearings, and (ii) limit the size of the bearing assembly components for a given tube and thus their ability to handle larger mechanical loads. As a result of the limits on the cooling of the bearing assembly, bearing temperatures of approximately 400.degree. C. are common in many high powered x-ray tube applications. Unfortunately, such high temperatures may deleteriously effect bearing performance. For instance, prolonged and/or excessive heating of the lubricant applied to each ball of a bearing can reduce the effectiveness of the lubricant. In addition, the lubricant can be boiled off causing contamination of the vacuum in the x-ray tube. Further, prolonged and/or excessive heating may also shorten the life of the bearings, and thus the life of the x-ray tube. For these reasons it is desirable to (i) reduce the amount of heat that reaches the bearings and (ii) effectively remove heat in the bearings, regardless of its source.
One known method to reduce the amount of heat passed from the anode assembly to the bearing assembly is to mechanically secure a heat shield between the anode and the bearing assembly. The heat shield serves to protect the bearing assembly by intercepting a portion of the heat radiated from the anode 130 in the direction of the bearing assembly. Unfortunately, heat shields are not able to completely protect the bearing assembly from heat transfer from the anode 130 and a portion of the radiated heat will be absorbed by the bearing assembly. Additionally, although the heat shield is useful in reducing heat transfer to the bearing assembly, the heat shield does not play a role in cooling or removing heat already absorbed in the bearing assembly. Furthermore, given that the bearing assembly is enclosed by the rotor, the bearing assembly is not able to efficiently radiate heat to the cooling fluid contained in the housing. Thus, once heat has been transferred to the bearing assembly it is not readily dissipated.
Another disadvantage caused by the limit on bearing temperature is that various processes during manufacture of the tube, such as exhausting and seasoning the tube, are deleteriously affected. Exhausting the tube is the process in which vacuum is drawn in the tube. The tube is operated with internal components at high temperatures while a vacuum pump is operatively attached to the tube. The rate at which gas is removed from the tube and the resulting final pressure of the tube are related to the temperature of the components, such as the anode, during exhaust. The higher the temperature of the component the more effectively the gas is removed from the tube and the lower the pressure of the tube after exhaust. The bearing temperature limit results in reducing the temperature at which the components, i.e. the anode, can reach during exhaust.
The current bearing designs also limit component temperature during seasoning. Seasoning is the process in which the tube is exposed to progressively higher voltages and power. This "burn in" procedure assists in making the tube more electrically stable at high voltages experienced during tube operation. During the seasoning process the anode target focal track is exposed to some of the highest temperatures that it will experience. During seasoning, the focal track of the anode outgasses and evolves gas molecules into the vacuum envelope, thereby raising the gas pressure. The evolved gasses are absorbed by a getter within the vacuum envelope. Again, the bearing temperature limit causes a reduction in the temperature of the internal components during seasoning of the x-ray tube.
In addition, with higher power and/or higher velocity rotating anode applications, it is desirable to maintain acceptable runout specifications while increasing any of (i) the size of the anode disks, (ii) the rate of acceleration of the rotating anode to operating velocity and (iii) the rotational speed of the x-ray tube around the patient in a CT gantry. These higher power and/or higher velocity applications will present increased thermal and mechanical loads on the bearings. Present designs of bearing assemblies have a number of components surrounding the bearings which limit bearing size. Some of the components include the bearing housing and induction motor rotor. As a result of the limited bearing size, the mechanical and thermal loads that current sized bearing assemblies can handle without compromising bearing life and runout specifications is limited.
Anode size is also limited by present anode and bearing assembly mounting structures. Many of these mounting structures support the x-ray tube in a cantilevered fashion in the x-ray tube housing. This mounting arrangement requires that the mounting structure have sufficient strength to resist deformation during operation. However, since the mounting structure is typically the only point at which heat is conducted through the bearing assembly into the cooling oil, it is desirable to make the mounting structure out of a material that is a good thermal conductor. Materials that are good thermal conductors typically are not as resistant to deformation under normal x-ray tube mechanical operating loads experienced in CT systems. These two requirements, high strength and good thermal conductivity, often dictate conflicting choices in a mounting structure materials. Designs are often a compromise that attempt to select a material that can perform both functions satisfactorily.
Therefore, it is desirable to provide an x-ray tube that provides for more effective cooling of the bearings. More effective cooling of the bearings permits higher x-ray tube component temperatures, e.g. anode temperatures during exhaust, seasoning, and tube operation. It is also desirable to provide larger bearings to handle greater mechanical and thermal loads for larger and/or high power x-ray tube applications while maintaining runout specifications.